Improve OS X build and docs

Improved the README with build requirements and links
Added initial credits for Info box on OS X
Improved Xcode build on OS X, should resolve SDL location better
Conditional compile for OS X to stop SDL setting app icon and
overriding the nicer high res image (may not be wanted on all
platforms)
Merge pull request #9 from andypiper/osx-improve

Improve OS X build and docs
Merge branch 'master' of git://github.com/fabiensanglard/chocolate_duke3D

# By Andy Piper
# Via Andy Piper (1) and fabiensanglard (1)
* 'master' of git://github.com/fabiensanglard/chocolate_duke3D:
  Improve OS X build and docs